Over the past several years, the catode ray tube has evolved as an important means for displaying information in computer terminals. This is because the cathode ray tube is well developed, cost effective device with fast writing and erasing speeds. Most display tubes have been of the monochrome type; however, recent needs have been for high resolution color displays in order to properly present the increasingly sophisticated and complex information generated by computers. Most of the commercially sold high resolution color display tubes have used delta electron gun and dot screen systems. When properly set up, such tubes have very good center and corner resolutions and good electron beam convergence. However, it is known that the convention circuitry required for a delta electron gun is not only costly, but also subject to drifts. Since display tubes are usually viewed at closed range, such convergence drifts are very undesirable.
It has been determined that a self-converged system using an inline electron gun, such as that disclosed in U.S. Pat. No. 3,772,554 issued to R. H. Hughes on Nov. 13, 1973, a self-converging yoke, and a dot screen provides improved display tube performance because of the elimination of convergence drift. However, the performance of the electron beam spot at the corners of the display tube tends to suffer because of the self-converging yoke. Since the corners are just as important as the center of the tube when displaying characters theron, there is need to improve display tubes and especially the inline electron guns therefor to improve performance at the corners of the tubes.